Armed pirates raided a tanker off the West African coast and kidnapped 23 sailors Wednesday, taking off with the vessel in waters that are increasingly at risk of piracy, an international monitoring group said.

The International Maritime Bureau, which tracks piracy worldwide, said pirates boarded the tanker as it idled about 62 nautical miles from Benin's capital of Cotonou. Pirates struck as the Cyprus-flagged vessel tried to transfer its cargo of crude oil to a Norwegian-registered ship, said Cyrus Mody, a manager at the bureau.

The pirates sailed off with the crew to an unknown location, Mody said.

The ship, called the Mattheos I, had a Filipino crew with Spanish, Peruvian and Ukrainian officers, said Serghios Serghiou, the director of Cyprus' Department of Merchant Shipping.

Serghiou said Cyprus authorities and the ship's Spanish management company had not been able to confirm Wednesday whether a hijacking took place.

"The ship sent out the initial security alert, but unfortunately, we haven't been able to communicate with the ship," Serghiou said.

A spokeswoman for Spain's Foreign Ministry said Spaniards accounted for less than five of the hostages. She spoke on condition of anonymity because of the sensitivity of the matter.

The pirates attacked the Norwegian ship at the same time, though the crew was able to lock themselves into a strong room and wait for the attackers to leave, Mody said.

Over the last eight months, piracy in the Gulf of Guinea has escalated from low-level armed robberies to hijackings and cargo thefts, according to the Denmark-based security firm Risk Intelligence. Last month, London-based Lloyd's Market Association, an umbrella group of insurers, listed Nigeria, neighbouring Benin and nearby waters in the same risk category as Somalia, where two decades of war and anarchy have allowed piracy to flourish.

West African pirates also have been more willing to use violence - beating crew members, and shooting and stabbing those who get in the way. Analysts believe many of the pirates come from Nigeria, where corrupt law enforcement allows criminality to thrive.

Those operating in the region have been warned not to stay too close to the shoreline and work only during daylight hours, Mody said. However, the attacks keep happening, as pirates in the region seem to favour the oil vessels now sailing through the waters.

"This is an area of risk," Mody said. "There's no doubt about it."

Analysts believe some of those oil tankers carry crude stolen from Nigeria's oil-rich southern delta, where thefts run into the hundred of thousands of barrels of oil a day.

The maritime bureau says Nigeria and Benin reported 18 pirate attacks in the first half of 2011. While smaller than figures attributed to Somali pirates, shipping industry officials say the number of attacks off Nigerian waters is underreported because some ships carry the illegal oil cargo and others fear their insurance rates will rise.

A spokesman for the Nigerian navy did not immediately respond to a request for comment Wednesday. Authorities in Benin could not be immediately reached.